"Long, long ago. Back when great wars were fought between the ponies and the Diomedians, nothing like the small scuffles of today but thousands upon thousands of each side staking their future on a single battle. During these battles immense amounts of magics were used by our unicorns and alicorns as well as their Diomedian counterparts. Such magic leaves traces, pile up enough traces and you get an entirely new creation, a creation influenced by the place of its birth. Since the sheer amount of magical residue would only occur in such pitched battles these creatures weren't given the best of influences. The hatred and fear of the living, the pain and despair of the dying, every negative emotion known to pony-kind in one steaming cesspit of hate. From these were born what we called Wraths. I'm not sure what they're called these days though, most of them were killed early on, they were stupid creatures driven by hate for everything around them and sustained by the same emotions that gave them life. As soon as they were 'born' they would take control of a nearby corpse, any kind of corpse, be it pony, diomedian, or something else. They would then rush around causing as much mayhem and destruction as they could in order to feed themselves until they were either killed or simply starved to death. Unfortunately for them they were rare creatures, and there's only so much a single badly mutilated corpse can do to a trained army complete with battlecasters. However one of these creatures made a mistake that none other had made before or has made since. It possessed a living being. Near death, delirious and might as well be dead, but still alive. It wasn't prepared to face any resistance on a mental front and was defeated easily. But in it's defeat it was bonded to it's host. The usual transformation that renders what was usually a corpse into a somewhat working creature still happened and brought the host fully to life, and as long as the Wrath is fed both will survive."
I smiled cheerfully and added, "I call him Negs."